Abstract
The Croton Watershed (New York State, USA) is a semi-urban region that provides 10% of the drinking water for the City of New York. Nonpoint source contamination in the watershed is a major concern for managers because the water supply is currently unfiltered water. Results are reported from three synoptic studies of surface water quality from 98 wetland-containing sub-catchments in the Croton Watershed designed to broadly characterize, at a reconnaissance level, the geochemical controls on water quality, in particular as it relates to wetlands. Total dissolved organic carbon concentrations in surface waters draining wetlands correlated well (average R2 of 0.93) with standard Gelbstoff (g440) color measurements, although there is very little correlation between dissolved organic carbon concentrations and wetland areas in the sub-catchments. This may be a potential indication of other sources of colored organic material. Concentrations of dissolved sodium and chloride, while related to road length, stochiometrically had more chloride than expected for pure road-salt dissolution. This offset is likely due to cation exchange and sorbtion of sodium by wetlands in the Croton watershed. The results show contamination in the Croton hydrologic system that should addressed in ongoing management policies and decision-making.
Highlights
The characterization of nonpoint source contaminants and chemical processes in large natural hydrologic systems is difficult because reaction rates, reagents, and pathways vary spatially and temporally
Results are reported from three synoptic studies of surface water quality from 98 wetland-containing sub-catchments in the Croton Watershed designed to broadly characterize, at a reconnaissance level, the geochemical controls on water quality, in particular as it relates to wetlands
The water chemistry results from the three synoptic samplings do not fully support the hypothesis that greater wetland area correlates with increased dissolved organic carbon (DOC) and color, but rather that the link between wetland coverage, DOC, and color in the Croton watershed is complex, perhaps because residence time for surface water in many of the small wetlands are too short to produce large amounts of DOC in the waters, compared to wetlands in other watersheds underlain by peat
Summary
The characterization of nonpoint source contaminants and chemical processes in large natural hydrologic systems is difficult because reaction rates, reagents, and pathways vary spatially and temporally. Heisig [8] first addressed nonpoint source contamination in the Croton reservoir system by studying the controls of baseflow in several sub-catchments He found that the surface water chemistry appears to be dominated by road salt dissolution. Nitrate concentrations are closely related to the density houses not connected to a municipal sewer system (i.e. having a septic-field system), and nitrate concentrations were lowered by riparian wetlands His studies were in largely urbanized areas of the watershed. The Croton watershed is notable for having extensive wetlands that cover 6% of the total watershed area [9] While these wetlands have important hydrologic functionality, many questions remain regarding their interaction with nonpoint source contaminants. We used a synoptic sampling approach over the Croton Watershed in an attempt to understand the function and interaction of wetlands on water chemistry, including water coloration and road salt contamination. Within a large watershed such as the Croton Reservoir, sampling many places for water quality parameters within local landscapes can link water quality changes with landscape change, broad hydrogeochemical processes and geospatial data tied to landscape changes [16]
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